Use of the vasodilator sodium nitroprusside to increase tumour temperature during loco-regional hyperthermia with a capacitive ring applicator in a rat tumour model.

The influence of sodium nitroprusside (SNP) induced hypotension (to a mean arterial pressure of 60 mmHg) on tumour and normal tissue temperature during hyperthermia (HT) was examined. Loco-regional HT was given to the calf of BD IX rats by external radiofrequency heating from a capacitive ring applicator. In experiments in rats with subcutaneous BT(4)An tumours, the mean tumour temperature increased by 0.49 degrees C from 42.36 to 42.85 degrees C, on average, during SNP-hypotension. This represented 58% of the increase in tumour temperature found in the same rats when the tumour circulation was stopped completely by sacrificing the rats. SNP-hypotension resulted in a decrease in mean muscle temperature from 41.73 to 41.23 degrees C. The temperature difference between the tumour and the underlying muscle thereby increased by approximately 1 degrees C, indicating that SNP can increase tumour temperature during HT without increasing the risk of heat-related damage to skeletal muscle. Experiments in rats without tumours were also done to further examine the effect of SNP-hypotension on muscle temperature under different treatment conditions (variation of radiofrequency energy deposition and water bolus temperature). It was found that SNP decreased the muscle temperature during HT in two experiments where the average muscle temperature was 42.1 and 42.6 degrees C, respectively. In an experiment where the muscle temperature was 43.0 degrees C, on average, before SNP infusion, the muscle temperature increased during SNP-hypotension. This finding indicates that SNP-hypotension during HT may increase the risk of skeletal muscle necrosis with muscle temperatures at this level.

Development of a regional hyperthermia treatment planning system.

A flexible and fast regional hyperthermia treatment planning system for the Coaxial TEM System has been devised and is presented. Using Hounsfield Unit based thresholding and manually outlining of the tumour, a 40 cm CT data set (slice thickness 5 mm) is segmented and down scaled to a resolution of 1 cm, requiring only 30 min. The SAR model is based on the finite-difference time-domain (FDTD) method. The number of time steps to achieve numerical stability has been determined and was found to be 7000. Various optimizations of the SAR model have been applied, resulting in a relatively short computation time of 3.7 h (memory requirements 121 MB) on a Pentium III, 450 MHz standard personal computer, running GNU/Linux. The model has been validated using absolute value(Ez) measurements in a standard phantom inserted in the Coaxial TEM Applicator under different conditions and a good agreement was found. Hyperthermia treatment planning in combination with the homemade visualization tools have provided much insight in the regional hyperthermia treatment with the Coaxial TEM Applicator.

Calculation of change in brain temperatures due to exposure to a mobile phone.

In this study we evaluated for a realistic head model the 3D temperature rise induced by a mobile phone. This was done numerically with the consecutive use of an FDTD model to predict the absorbed electromagnetic power distribution, and a thermal model describing bioheat transfer both by conduction and by blood flow. We calculated a maximum rise in brain temperature of 0.11 degrees C for an antenna with an average emitted power of 0.25 W, the maximum value in common mobile phones, and indefinite exposure. Maximum temperature rise is at the skin. The power distributions were characterized by a maximum averaged SAR over an arbitrarily shaped 10 g volume of approximately 1.6 W kg(-1). Although these power distributions are not in compliance with all proposed safety standards, temperature rises are far too small to have lasting effects. We verified our simulations by measuring the skin temperature rise experimentally. Our simulation method can be instrumental in further development of safety standards.

ESHO quality assurance guidelines for regional hyperthermia.

The Technical Committee and the Clinical Committee of the ESHO evaluated the experience of the institutes which are active in clinical regional hyperthermia using radiative equipment. Based on this evaluation, QA guidelines have been formulated. The focus of these guidelines lies on what must be done not on how it should be done. Subjects covered are: treatment planning, treatment, treatment documentation, requirements and characterization of equipment, safety aspects, hyperthermia staff requirements and instrumentation for quality assurance.

Quality assurance for radiofrequency regional hyperthermia.

Today most treatments with regional hyperthermia are applied using radiofrequency systems with 'focus' steering by amplitude and phase control. This paper deals with quality assurance procedures developed to ensure controlled and safe treatments in such systems. Our results show how the deviations between requested and observed phase and amplitude vary with frequency, and how these deviations depend on both the geometry of the object (phantom) inside the system and the power level applied. The results also indicate that the investigated systems' internal quality assurance procedures were inadequate and that additional procedures should be applied. Since the system parameters depend on patient and treatment specific conditions it is concluded that there is a need for QA measurements before or during treatment. This paper deals specifically with the commercial BSD-2000 system from BSD Medical Corp. in Salt Lake City, Utah, as installed in Bergen, but the procedure outlined can be applied to other phase and amplitude-controlled RF-RHT systems with only minimal adjustments.

Radio-frequency ring applicator: energy distributions measured in the CDRH phantom.

SAR distributions were measured in the CDRH phantom, a 1 cm fat-equivalent shell filled with an abdomen-equivalent liquid (sigma = 0.4-1.0 S m-1; dimensions 22 x 32 x 57 cm) to demonstrate the feasibility of the ring applicator to obtain deep heating. The ring electrodes were fixed in a PVC tube; diameter 48 cm, ring width 20 cm and gap width between both rings 31.6 cm. Radio-frequency energy was fed to the electrodes at eight points. The medium between the electrodes and the phantom was deionised water. The SAR distribution in the liquid tissue volume was obtained by a scanning E-field probe measuring the E-field in all three directions. With equal amplitude and phase applied to all feeding points, a uniform SAR distribution was measured in the central cross-section at 30 MHz. With RF energy supplied to only four adjacent feeding points (others were connected to a 50 omega load), the feasibility to perform amplitude steering was demonstrated; SAR values above 50% of the maximum SAR were measured in one quadrant only. SAR distributions obtained at 70 MHz showed an improved focusing ability; a maximum at the centre exists for an electric conductivity of the abdomen-equivalent tissue of 0.6 and 0.4 S m-1.

Deep heating using a movable applicator phased array hyperthermia system. A preclinical feasibility study.

A preclinical evaluation of the 'movable applicator phased array hyperthermia system' was performed. The system employs four coherent applicators enabling power steering by amplitude and phase control. This concept has already been used in other systems, but the combination with a compact applicator design and easy movement of applicators has not been used before. The paper contains a description of the system and a verification of its performance using quality assurance tests with scanned E-field measurements. A clinical simulation was performed in pig to address the clinical feasibility of the system. The target volume was the left kidney. Two heating sessions, with and without occluded blood-flow to the kidney, were performed. In the low-flow experiments a temperature of 48 degrees C and 46 degrees C was obtained in the upper and lower pole of the kidney respectively. For the high-flow experiment the temperature in the upper pole was 48 degrees C.

Multi-applicator hyperthermia system description using scattering parameters.

Hyperthermia systems using electromagnetic phased arrays have often been investigated from a SAR distribution point of view. One of the problems in these systems is that the achieved SAR distribution is different from the intended one because of changes in the generator signals due to mutual coupling between applications. The use of circuit theory and S-parameters in the description of an N-applicator phased array system is introduced in this paper, and a compact matrix equation giving excitation as a function of generator signals is derived. Measurement of the S-parameters is discussed using a phased array deep heating system (Danish Hyperthermia Foundation (DHF)) as an example. In a phantom experiment a significant improvement in the SAR distribution is demonstrated when the developed method is applied.